On OSX dont enable LLD and warn (#7474)

Disabled LLD on OSX
diff --git a/build_tools/cmake/iree_setup_toolchain.cmake b/build_tools/cmake/iree_setup_toolchain.cmake
index ea6be81..fd63cd0 100644
--- a/build_tools/cmake/iree_setup_toolchain.cmake
+++ b/build_tools/cmake/iree_setup_toolchain.cmake
@@ -17,7 +17,7 @@
   set(IREE_USE_LINKER "lld")
 endif()
 
-if(IREE_USE_LINKER)
+if(IREE_USE_LINKER AND NOT APPLE)
   set(IREE_LINKER_FLAG "-fuse-ld=${IREE_USE_LINKER}")
 
   # Depending on how the C compiler is invoked, it may trigger an unused
@@ -53,4 +53,6 @@
     message(FATAL_ERROR "Compiler '${CMAKE_C_COMPILER}' does not support '${IREE_LINKER_FLAG}'")
   endif()
 
+elseif(IREE_USE_LINKER)
+    message(STATUS "On Apple OSX use system linker")
 endif()